Recently, there has been a surge of interest in ecology for neutral models of coexistence. In neutral models of coexistence, niche differentiation among species is removed from consideration and species are considered to be equivalent. However, fundamental properties of population dynamics (e.g., probabilities of birth, death, and immigration) and interspecific competition play an important role in determining coexistence. Many of the patterns of coexistence and diversity, species-area relationships, and the distribution of abundances among species in the real world can be explained through neutral models that do not include niche differentiation among species. Frequency-dependent natural selection can also generate coexistence between species that are apparently neutral (i.e., there is no niche differentiation between the species). In this situation, when species are rare (in low-abundance), they adapt to a high frequency of interspecific competitive interactions, thus providing for their persistence. When species are common (in high-abundance), they adapt to a high frequency of intraspe-cific competitive interactions, as they are more likely to encounter conspecifics in the habitat in which they are abundant. Under these conditions, the species are said to have evolved 'pseudo-neutrality'. Nevertheless, the
Table 1 Some models and explanations of processes that promote coexistence among species and individuals within habitats
Recruitment limitation Niche diversification Tradeoffs
Competitive networks Predation limitation
Intraspecific aggregation Asynchronous life histories
Available space occupied at random from larval/ propagule species pool
As above, but habitat occupation limited by patchy supply of larvae/propagules Many available microhabitats promote coexistence
High performance in one life-history feature at the expense of reduced performance in another
No species is dominant over all others
High predation (esp. on common prey-switching)
promotes coexistence Physical factor(s) that create space for colonization/
replenishment Species undergo higher intraspecific than interspecific competition
Seasonal resource use asynchronous among species, promoting coexistence (e.g., reproductive cycling, population irruptions) No niche diversification among species, probabilities of birth, death, and immigration and interspecific competition determine coexistence
Species with dispersive larvae/propagules (e.g., coral reef fishes) occupying space at random
Marine benthic organisms esp., at edge of their range
Different plant parts used by invertebrates as a resource
Plants either produce many small seeds (good colonizing ability) or few large seeds (good competitive ability)
Predation on lower rocky intertidal marine environments
Storm gaps in rainforest canopies
Patchily distributed and aggregated plant populations of different species
High diversity of plant species in rainforests problem is that a community theory that accounts neither for species differences nor for patterns of species distribution across real physical geography must be incomplete.
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